In 1981, President Ronald Reagan claimed that trees produce more air pollution than automobiles, fueling a spate of jokes about “killer trees.” He was mostly wrong, but not completely.

It is true that forests emit volatile organic compounds and reactive hydrocarbons such as isoprene, a chemical that contributes to air pollution. Although isoprene is harmless by itself, the gas reacts with other substances in the atmosphere to form certain types of fine aerosols and ground-level ozone. (Ozone near the ground causes health problems, damages crops, and contributes to climate change. In the stratosphere, it is beneficial because it prevents harmful ultraviolet rays from reaching Earth’s surface.)

“What Reagan neglected to indicate is that unhealthly levels of ozone wouldn’t form without nitrogen oxides (NOx), pollutants emitted when gasoline and coal are burned,” explained Bryan Duncan, an atmospheric scientist at NASA’s Goddard Space Flight Center and the deputy project scientist for the Aura satellite. Since sunlight is ubiquitous and forests emit large quantities of volatile organics in the summer, it is the amount of nitrogen oxide that determines whether ozone forms over cities on hot summer days. That NOx is most often supplied by vehicles.

In the 1970s and 80s, the United States spent billions trying to limit hydrocarbon emissions, but such efforts had little impact on ground-level ozone because forests were providing plenty of natural hydrocarbons. Research conducted by William Chameides of Georgia Tech underscored the futility of trying to limit hydrocarbons while ignoring nitrogen oxides.

“Hydrocarbons from forests are so common and widespread in the eastern U.S. that their presence simply overwhelms anything happening with anthropogenic hydrocarbons,” Duncan said. “The only realistic way to try to limit ozone formation is to do something about nitrogen oxides, which is what has been done in the U.S. over the last several decades. As a result, surface ozone has declined. Not to mention that forests are a natural and crucial part of the environment. Cutting down trees just doesn’t make any sense.”

While satellites cannot measure isoprene emissions from forests directly, they can measure formaldehyde (HCHO), a gas that forms as a result of oxidizing isoprene emissions and as a byproduct of fires. The map above, based on data collected by the Ozone Monitoring Instrument (OMI) on the Aura satellite, shows the global distribution of formaldehyde in September 2013.

The highest concentrations of formaldehyde were present over the Amazon rainforest, in southeastern Africa, and in the southeastern United States. While isoprene from forests was the primary source of the formaldehyde in these areas, observations from the Moderate Resolution Imaging Spectroradiometer (MODIS) show that fires, most likely crop burning, contributed to the high levels in these areas as well.

Editor’s Note: Ten years ago, the Aura satellite launched from Vandenberg Air Force Base in California. Since then, Aura has made major contributions to our understanding of Earth’s atmosphere. To read more about Aura’s accomplishments and discoveries, view this slideshow.